Our investigation into the morphogenesis of bacteriophage will be continued, with emphasis on three aspects: (1) The detailed mechanisms of T4 phage tail fiber assembly and attachment, (2) the role of the host cell in viral assembly, and (3) characterization of intermediates in DNA packaging and other capsid-related structures during T7 phage maturation: (A) The assembly and attachment of T4 tail fibers requires four proteins, under phage genetic control, which are not found in the completed structure. Biochemical and genetic approaches will be followed in attempts to understand the mechanisms by which these proteins promote or direct the assembly of the structural protein components of the tail fibers. (B) Bacterial mutants have been isolated which block the normal process of T4 assembly, or which allow the assembly of mutant phage that are defective in a normally essential gene. The existence of these mutants demonstrates the involvement of host-cell factors in viral morphogenesis. We shall continue our genetic, physiological and ultrastructural analysis of these mutants with the goal of understanding the roles of these factors in normal cell function (if any) and in phage assembly. (C) Capsid-related structures isolated from T7-infected cells will be further characterized by ultracentrifugation, polyacrylamide gel electrophoresis, electron microscopy, and low ange X-ray scattering with the goal of understanding the process by which DNA becomes packaged in the phage head during viral maturation.